All useful ferrous-base and nickel-base corrosion-resistant alloys have generally been formulated from the same group of chemical elements, and all important advances in the field have been based upon the discoveries of new and useful proportions of these same chemical elements. Each of these types of alloys contains at least two of the most important or useful elements, iron, nickel, chromium, molybdenum and manganese, along with at least very small amounts of the generally undesirable carbon, phosphorus and sulfur. Some formulations further contain one or more of the group, copper, nitrogen, silicon, columbium, titanium, and, rarely, tungsten or aluminum. Those additional elements that have been explored but which have never achieved any significant usage in the field are calcium, magnesium, zirconium, beryllium, yttrium, boron, antimony, platinum, palladium, tantalum, lead, selenium, tellurium, cerium, lanthanum and mixtures of rare earth elements.
The austenitic stainless steels, containing a minimum of about 8% nickel and about 18% chromium, are several times more widely employed by tonnage than all other corrosion resistant alloys combined. They are the most resistant of ordinary stainless steels to industrial atmosphere sand aqueous acidic media except under strongly reducing conditions. They tend to be passive in media with a pH in excess of 3.0 and in oxidizing environments unless they contain undissolved chromium carbides and are employed in certain media. They are also generally passive in solutions at a pH of 2 to 10 and temperatures over about 150.degree. F. unless chlorides are present. grades of these so=called 18% Cr-8% Ni type stainless steels have been developed for various applications by the inclusion of columbium (niobium), titanium or molybdenum or by maintaining carbon levels of about 0.03% maximum.
Ferritic, martensitic and precipitation-hardened grades of stainless steels have been employed to a much lesser extent to achieve special mechanical properties or because they are cost-effective in less demanding corrosion situations than those met by the 18% Cr-8 % Ni family of steels.
Some of the most typical characteristic properties of the 18% Cr-8% Ni type stainless steels are low yield strength in the annealed condition, good weldability, moderately poor machineability, high coefficient of thermal expansion, low coefficient of thermal conductivity, low hardness, non-magnetic face-centered-cubic matrix crystal structure, high tensile elongation, very high toughness and impact strengths at all temperatures, and pronounced tendency to harden and strengthen with cold or warm working, such as in rolling or extensive forging.
Two of the most undesirable characteristics of austenitic stainless steels are their low yield strengths, unless work-strengthened, and nickel contents, generally of 8% or higher. As to nickel content, nickel is a moderately scarce element in the earth's crust and not present in any known ore deposits in the United States. Nickel is far more expensive than the many other constituent elements, such as iron, chromium, molybdenum, silicon, manganese, copper and tungsten.
Accordingly, there have been extensive attempts to find substitute alloys of lower or no nickel contents that would provide comparable corrosion resistance with equal or perhaps somewhat superior mechanical properties to the 18-8 type alloys. These attempts have included high manganese steels coupled with fractions of a percent of nitrogen, very high purity ferritic steels, and, more recently, the duplex stainless steels. While each of these three types has found application, each has presented one or more problems as an 18% Cr-8% Ni stainless substitute.
The most promising of these types has been the duplex group of approximately half-ferritic half-austenitic matrix crystal structure stainless steels containing variously 22% to b 26.5% Cr, 4.8% to 10% Ni, 1.5% to 4.5% Mo, 0.0%-2% Cu, 0.15% to 0.25% N, 0.4% to 1.7% Si, 0.8% to 2% Mn and the balance essentially iron.
Compared to the 18% Cr-8% Ni types of stainless steels, the commercial grades of duplex alloys may be cast or wrought, have much higher yield strengths and hardnesses, lower thermal expansion and higher thermal conductivity, while maintaining adequate toughness and ductility when properly heat treated. Their main disadvantages have been problems encountered in fabrication and the very high temperature solution heat treatments required to secure the desired matrix structures. When properly heat treated, they have generally good resistance to intergranular pitting and crevice and galvanic corrosion as well as to stress corrosion cracking and erosion-corrosion.
The relatively high molybdenum content of present day commercial grades of duplex stainless steels (usually 1.5% to 4.5%) tends to cause the formation of hard, brittle, highly-corrodable sigma phase under certain conditions of heat treatment. The high molybdenum content along with the comparatively high chromium content also causes the known duplex stainless steels to freeze with an entirely ferritic structure during weld solidification. Austenite is formed through a solid-state phase transformation during post-solidification cooling. The result is an uneven division of nickel, chromium, molybdenum and nitrogen between the two phases after welding in structures that are much too large for post-weld heat treatment, such as pipe lines and large tanks. Consequently, there is a reduction in corrosion resistance at the weld.
It is obvious that any proposed duplex alloy of greater than about 8% Ni does not constitute a potentially lower nickel content substitute for 18% Cr-8% Ni stainless steels. Also, in alloys of the order of 30% to 35% Cr, greater than 3% Mo and large amounts of silicon or aluminum, there is a greater potential for sigma formation and poor welding qualities than in the present commercial duplex alloys. Alloys of greater than about 4% Mn content present special melting and casting problems and tend to form large amounts of primary delta ferrite and ultimately sigma phase when chromium levels exceed about 18%. Silicon and molybdenum also tend to promote the formation of sigma phase. While silicon content is held to low values in duplex stainless steels, molybdenum contents generally vary from about 1.5 to 4.5%. This in part accounts for their strong tendency to high hardness, low tensile elongations and fabricability problems even with high temperature solution annealing and rapid cooling. Also, manganese contents greater than about 2% tend to reduce resistance to local corrosion in chloride-bearing solutions.
As noted above, the main drawbacks of the current duplex stainless steels as substitutes for the 18% Cr-8% Ni family of steels have been fabrication and welding problems, moderately high hardnesses and a significant reduction in tensile elongation for the gains in yield strength obtained along with the required high temperature heat treatments necessary to avoid sigma phase or other undesirable structures. In general, the commercial duplex alloys have required solution heat treatments of from about 2000.degree. (1093.degree. C.) to 2260.degree. F. (1238.degree. C.) followed by drastic oil or even water quenching.
Accordingly, in spite of all prior efforts there still remains a need for alloys which are much closer in chemical properties to 18% Cr-8% Ni stainless steels but of reduced nickel content and of approximately equal or superior mechanical properties.